It is known that most of the perspiration of the foot originates at the interface between the sole of the foot and the sole of the shoe.
On this interface, most of the sweat is unable to evaporate and accordingly condenses on the plantar insert on which the foot rests; the remaining part of the sweat that does not condense (a very small fraction) evaporates through the upper.
This phenomenon is particularly conspicuous in shoes which have a rubber sole; in these cases, vapor permeation through the sole is in fact prevented completely (in the case of soles with a leather tread, a small percentage of sweat is still able to evaporate through the tread).
In order to solve this problem, shoes with soles which are waterproof and vapor-permeable have now been devised for several years.
Their purpose is to allow the escape of the water vapor produced by the foot with perspiration.
One of these solutions is disclosed for example in U.S. Pat. No. 5,044,096 and EP 382904, which relate to a vapor-permeable sole which consists of two layer-like portions, respectively an upper portion and a lower portion of the sole, which are made of rubber or other synthetic material and are provided with through holes, and in interposing between said portions a membrane which is waterproof and vapor-permeable and is connected perimetrically and hermetically to the two portions so as to not allow water infiltrations.
The need to prevent foreign objects of a certain size from being able to arrange themselves inside the holes, with the possibility of damaging the membrane, forces the use of microperforations in the lower portion provided with the tread; the use of microperforations leads to a reduction of the potential vapor permeation area.
Moreover, the microperforations tend to become clogged easily, consequently reducing the actual vapor permeation area.
One improvement is provided in U.S. Pat. No. 5,983,524 and EP 858270, which disclose a shoe provided with a rubber sole which has through holes and a mid-sole composed of a waterproof vapor-permeable membrane and a layer for protecting the membrane, typically a layer of felt.
Although this invention improves protection against external elements, it forces the use of holes measuring 1.5-2.0 mm, which are spaced enough to avoid compromising the structural strength of the tread.
These solutions are not particularly suitable for the disposal of the large amounts of water vapor that form, with perspiration, inside the shoes of individuals who have higher-than-average perspiration.
Excessive foot perspiration can also occur in the case of extremely hot and humid climates and if shoes are used for sports activities.